fix example dropping sink immediatly halting playback

Author: dvdsk

src/lib.rs

@@ -48,7 +48,8 @@
 //!
 //! // Load a sound from a file, using a path relative to Cargo.toml
 //! let file = BufReader::new(File::open("examples/music.ogg").unwrap());
-//! rodio::play(&stream_handle.mixer(), file).unwrap();
+//! // Note that the playback stops when the sink is dropped
+//! let sink = rodio::play(&stream_handle.mixer(), file).unwrap();
 //!
 //! // The sound plays in a separate audio thread,
 //! // so we need to keep the main thread alive while it's playing.